In the current powerline network system (e.g., HomePlug®), the amateur radio frequency bands are eliminated by notch filters to prevent interference. However, the other frequency bands are not eliminated. Interference could occur when a user listens to, for example, a shortwave radio broadcast program. This is one of the main reasons why some shortwave radio broadcasters are opposed to use of powerline communications (PLC) networks.
The HomePlug® is a powerline network standard that is widely used in the United States of America. HomePlug® PLC networks use Orthogonal Frequency Division Multiplex (OFDM). Transmitted data are split into 70 to 80 sub-carriers spread from 1.8 to 20 MHz and sent to their destination over the power line. A power line is under very harsh condition and transmission conditions can vary every moment. For reliable transmission, the transmitter and the receiver often exchange a tone map. The receiver receives test tone signals from the transmitter and measures signal-to-noise ratio (SNR) for each sub-carrier. Based on SNR, sub-carriers and modulation schemes (ex. Binary Phase Shift Keying or Quadrature Phase Shift Keying) are selected. This sub-carrier and modulation information (tone map) is sent back to the transmitter. The transmitter performs transmission based on the tone map.
The sub-carriers that would ordinarily fall within the amateur radio bands (ex. 7, 10, 14, 18 MHz) are not used in the HomePlug® standard. Fixed notch filters are applied to these frequency bands to reduce noise emission level so as not to interfere with amateur radio communication. However, other frequency bands which are shared with other users of the spectrum may be problematic, since the PLC may generate and radiate noise in these bands which can interfere with communication. One example is shortwave radio broadcasts such as Voice of America (VOA), which uses 6.16 MHz. When a PLC network is in use, a user may have to contend with interference that inhibits the user from being able to listen to VOA broadcasts without PLC noise.